Motor bearings

Reprint from Global Cement Magazine October 2007

ABB knows the most common bearing diffi culties and how to minimise them

Rolf Hoppler & Reinhold A Errath ABB Switzerland Ltd

Motor bearings: ‘the bearing necessities’Motor bearings are of high importance in drive systems, so their selection and handling should be taken seriously. Rolf Hoppler and Reinhold Errath evaluate the types of bearings used in the cement industry, off ering their hints and tips for avoiding problems with lubrication, greasing, motor cabling, installation, alignment, bearing currents, temperature and vibration.

2 globalcement MAGAZINE October 2007

Not all bearings are suitable for every application; a universal, all-purpose bearing does not exist. Th e

choice of bearing arrangement is based on the following qualities:• load carrying capacity in the axial and radial direction• overspeed and duration• rotating speed• bearing lifeTh e size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability.Other factors must also be taken into consideration, such as operating temperature, dirty and dusty environ-mental conditions, and vibration and shocks aff ecting bearings in running and resting conditions.

Deep groove ball bearingsTh ere are many types of bearings on the market, each with diff erent characteristics and diff erent uses. Deep groove ball bearings are the most common type of bear-ing, and can handle both radial and thrust loads. Due to their low-frictional torque, they are suitable for high speeds.

In a ball bearing, the load is transmitted from the outer race to the ball and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it to spin very smoothly. Th is also means that there is not very much contact area holding the load, so if the bearing is overloaded, the balls can deform, ruining the bearing.

Cylindrical roller bearings Th ese roller bearings are used in applications where they must hold heavy radial loads. In the roller bearing, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. Th is spreads the load out over a larger area, allowing the bearing to handle much greater radial loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading.

Angular contact ball bearings Angular Contact ball bearings have raceways in the inner and outer rings which are displaced with respect to each other in the direction of the bearing axis. Th is means that they are suitable for the accommodation of

combined loads such as simultaneously acting radial and axial loads in vertical machines.

Spherical roller thrust bearing In Spherical Roller thrust bearings, the load is transmit-ted from one raceway to the other at an angle to the bearing axis. Th ey are suitable for the accommodation of high axial loads in addition to simultaneously acting small radial loads. Spherical roller thrust bearings are also self-aligning.

Sleeve bearings Th e life of a sleeve bearing is practically infi nite, pro-vided that its operation remains within the specifi ed conditions. Motors have sleeve bearings at both ends. Th e bearing on the D end is the guide bearing and means that it can tolerate a limited non-axial force. Th e bearing on the N drive end is isolated.

Th e bearings are rigidly mounted to the end shield of the machine. Th e bearing housing is made of cast iron. Tapped holes for thermometer, oil inlet and outlet and oil level are provided on both sides of the housing. Th e bearings are lubricated by hydrodynamic lubrication, which can be of a self-lubricating or oil circulation type.

Th e bearing shells are spherically-seated in the hous-ing. Th e oil fl ow of self-lubricated bearings is guaranteed by the central arrangement of the oil ring. Th e precise shell seating also provides good heat transfer between the bearing shell and the housing. Th e shell consists of a steel body lined with white metal. Bearings with a cir-culating oil system are also equipped with an oil ring, to allow for safe running during a coast stop of the motor, in case of a power failure.

Flange mounted sleeve bearingsFlange mounted sleeve bearings are used for machines with a shaft height of up to 1120mm. Machines with bearings of this type are quick and easy to align. Th e

Top: Deep groove ball bearings.

Above: Bearing selection graph.

In the cement industry, the two

main families of bearings are

anti-friction bearings (for lower

power ratings) and sleeve bear-

ings (for higher power ratings).

Above right: Angular contact

ball bearings.

Below: Cylindrical roller

bearings.

air gap between stator and rotor comes from the factory already adjusted, and does not need any further adjust-ment on site during installation.

Foot mounted sleeve bearings Foot mounted sleeve bearings are mounted on a ped-estal. Th e pedestal can either be integrated in the stator frame, or can be mounted separately. If it is integrated with the stator frame it is easy and fast to align.

Axial fl oating and forcesSleeve bearings are designed to tolerate only limited continuous axial loads. Th e axial load is absorbed by the plain, white metal-lined thrust faces. Precautions must be taken to prevent excessive axial loads.

Standard motors are designed for a rotor axial fl oat of +/-8mm from the mechanical centre in the middle of the rotor end fl oat limits. Th e magnetic centre does not need to be in the same position as the mechanical centre, but is located between the end fl oat areas. Th e magnetic centre is permanently marked on the shaft with a groove. When the motor is running, the rotor will take the position of the magnetic centre.

Th e sleeve bearing is not designed to withstand any axial forces from the driven machine. All axial forces must be carried by the driven machine. Th e coupling must be of limited axial fl oat type, and the limit must be smaller than the rotor axial fl oat.

Radial forcesOnly radial forces from the coupling are allowed. If any additional radial forces are expected, they will have an infl uence on the bearing design.

Balancing Aft er manufacturing, the rotor is not balanced because of manufacturing tolerances. Dynamic imbalance is caused by unevenly distributed masses around the rotor. As centrifugal forces increase with the square root of the speed, any imbalance will lead to strong asymmetrical radial forces. Th ese forces cause swinging movements in the shaft of the rotor and can lead to vibrations which could harm the bearings and the rotor itself. To avoid this eff ect, all motors leaving the factory are dynamically balanced. Balancing can be made with half-key, full-key and the coupling half. Th e rotor balancing method is marked on the shaft end.

VibrationTh e vibratory stresses in the motors, connected ma-chine parts and foundation must be reliably kept within the specifi ed limits. Any violation of these limits is detrimental to the lifetime of the bearings, beside other negative eff ects. Care also has to be taken when the rotor shaft is passing through a region of resonance. Th e allowable vibration is defi ned in IEC 34-14. For motors above 300kW, the following values are valid:

Motor running (not coupled):<500 <n < 1800 < 1.8 mm/sec<1800 <n < 3600 < 2.8 mm/sec

Usually, the customer is responsible for the motor foundation; however, the respon-sibility for it has to be defi ned. Th e foundation should not only be rigid enough to withstand short circuit forces, but also the natural frequencies of the system motor and foundation should not coincide with the rotational frequency of the machine, or with any of its harmonics. Th e foundation construction should not cause any substantial de-crease in critical speed for the operation of a motor.

Insulated bearings In general the non-drive-end bearing is insulated. If the motor is operating on a frequency converter, the insulation of the non-drive-end bearing is a must, because of the existing bearing currents. Th e insulation has to be checked aft er bearings have been replaced.

Common bearing problemsAnti-friction bearings usually have a life of over 100,000 hours, corresponding to an active life-time of about 12-15 years. Sleeve bearings should have an infi nite lifetime. Th is requires that the bearings are of correct size, dimension and type, and are also well maintained. If not, the lifetime will be signifi cantly shorter. We will now summarise the most common bearing problems.

VibrationRoller bearings are easily damaged from vibration when the motor is not running, so the rotor is locked during transportation. Ball bearings can sustain more vibration than roller bearings when not running. Both types can only withstand single and infrequent shocks of 2-3g without sustaining damage; shocks of greater magni-tude should obviously be avoided.

Sleeve bearings can sustain single and infrequent shocks of 3-5g. Again, the rotor is fi xed axially during transportation: don’t forget to unlock it before energizing

globalcement MAGAZINE October 2007 3

Left: A fl ange-mounted sleeve

bearing

Motor coupled to the load Warning > 7 mm/sec

Motor coupled to the load Trip > 9 mm/sec

Rule of thumb for vibration protection settings

Support class Validation RMS velocity (mm/sec)

RigidA/B B/C C/D

2.3 4.5 7.1

FlexibleA/B B/C C/D

3.5 7.1

11.0

Validation A: Newly commissioned motors should be in this range

Validation B: Acceptable for long term operation

Validation C: Normally considered as unsatisfactory for long term continuous operation, but operation is permitted for a limited time period.

Validation D: Standard motors operating in this vibration range are likely to sustain severe damage.

Vibration validation ranges

Below: Bearing insulation,

formed by a shrink-fi tting insula-

tion layer in the shield.

the motor. Vibration in mo-tors is normally caused by• Unbalanced loads, like fans mounted on unstable base frames, can provoke heavy vibration• Operating equipment near resonance points, especially when adjustable speed drives are used, can provoke heavy vibration• Lack of uniformity in the magnetic fi eld

• Partial short circuit in the windings• Damaged bearings• Excessive axial forces• Radial or axial misalignment between the motor and the load machine• Incorrect balancing of coupling half.

Lubrication and greasingTh e purpose of lubrication is to prevent direct metallic contact between the various rolling and sliding ele-ments. Th is will be achieved by the formation of a thin oil fi lm between the two surfaces. Th e greasing interval recommended by the bearing supplier has to be strictly followed. Only lubricants which are recommended by the supplier of the motor, or those with very similar characteristics to the recommended lubricant, should be used. Th e use of other lubricants will lead to problems, sooner or later. Problems caused by the application of grease or lubrication can also shorten the lifetime of the bearings in the following situations.

Causes of high bearing temperature• Over-greasing the bearing, which forces the balls to push through excess grease as they rotate, leading to a sharp temperature rise• Too little grease in the anti-friction bearing, or too little oil in the sleeve bearing• Too low, or too high temperature, of the oil cooling water for the sleeve bearing• Excessive radial forces on the bearings• Tension of V-belt drives is too high• Ambient temperature is too high (for instance, the drive motor below the kiln has insuffi cient heat protection)• Friction of the bearing sealing (bad shaft seals leading to loss of grease or oil)• Using a low-temperature grease which does not provide adequate viscosity at normal operating temperatures• Mixing incompatible greases, which can reduce the consistency of the grease and possibly the overall viscosity.

Motor specifi cation problems• Radial bearing load is not correctly specifi ed• Axial load is not correctly specifi ed, especially in the kiln drive, where the motor is mounted with a slope

between 2 and 5 degrees. Th e result will be higher tem-perature and shorter lifetime of the bearing• Existence of bearing currents not specifi ed• In motors in the megawatt range, specify a grounding ring with grounding brush to avoid bearing currents.

Motor noiseBasically, sleeve bearings don’t make noise at all, and Ball bearings are more noisy than Roller bearings. Damaged bearings create higher motor noise as well; anti-friction bearings, without adequate grease, also create higher motor noise.

Motor storageIf motors have to be stored before installation it has to be done appropriately in order to avoid damage. Aft er a lengthy storage period, a careful inspection is gener-ally recommended. Any corrosion must be removed. If the shaft bears imprints on the lower half, it must be replaced.

Th e storage location should be free of vibration, shock and corrosive gases. If stored in the vicinity of the sea, the entire motor (not just the bearings) has to be protected from salt water and humidity.

Anti-friction bearingsAnti-friction bearing have to be well lubricated through-out the duration of the storage period. In longer storage periods, the lubrication condition needs to be checked from time to time. Depending on the ambient storage temperature of the motor, a fully-penetrating grease lubricant with a wide temperature range, for instance -30 to + 100oC, has to be used.

To keep the anti-friction bearings in good condition, the rotor should be turned about 10 revolutions every two months. Before turning the rotor, the transportation lock has to be removed, and aft er turning, the transpor-tation lock has to be fi xed again. Remember not to fi x the lock too tight (about 10Nm torque should suffi ce), because this could harm the bearings.

Sleeve bearings in storageMotors with sleeve bearings are generally delivered without oil. During storage, it is important to make sure that the surface of the bearing is always covered with Tectyl. Th e Tectyl should be sprayed into the bearing through the fi lling hole if the storage time is longer than one month. If storing for much longer than one month, the treatment should be repeated every six months. If the storage is longer than two years, the bearings should be dismantled and treated with corrosion protection. Before the stored motor is used, the bearings have to be fi lled with oil of a high quality and a type recommended by the motor supplier.

Summary of reasons for bearing failuresImpending bearing problems are preceded by a change in its behaviour. Early indications for potential prob-lems are increases in temperature, vibration levels or noise levels of the motor.

4 globalcement MAGAZINE October 2007

Above: Common reasons for

bearing failures. Most of the

known failures could be avoided

if a proper bearing diagnostic

and supervision system was

in place, if the measurements

which such a system provides are

interpreted correctly.

In a correct installation, and under adequate super-vision, the temperature and the vibration can be easily visualised by trend logs. Th e noise level, however, can only be detected by the maintenance staff during rou-tine checks. If potential problems are not recognised and analysed promptly, or if incorrect diagnoses of the problems are made, sooner or later it will lead to a bear-ing problem.

Bearing currentsBearing currents have been recognised for a long time. In the 1920s, the currents were a consequence of asym-metrical stator windings. As the fabrication of windings and motors improved, these currents became less and less signifi cant, and today are no longer important.

On the other hand, in recent times, the growing use of frequency converters with PWM technology has brought back the bearing current discussion again. Modern AC Converters have as their motor output, a high du/dt (voltage gradient) combined with a high switching frequency. Th is results in the sum of the three-phase voltages not being zero any more, as it is in a three-phase network. Th e so-called common volt-age depends on the intermediate circuit DC voltage and the switching frequency. Without considering counter measures for these eff ects, a motor bearing can be destroyed within a few months of operation. If such a common mode voltage is present, there might be dif-ferent dominant root causes. Th is voltage always tries to generate a current fl ow.

Damage due to current dischargeBasically, the damage is always caused by partial dis-charge (electrical discharge machining, or EDM). Th e oil fi lm between the race and the ball acts as a dielectric (capacitor), which is charged by the bearing currents. As soon as the voltage level is high enough it will be discharged by short circuit. Such periodic discharging will erode the metal.

Th e speed of the rotor infl uences the erosion. Higher revolutions create a thicker oil fi lm and therefore more capacity. Because of this, the voltage, until a fl ash-over occurs, is higher and the damage greater. Rotors rotat-ing at lower speeds, on the other hand, have a thinner oil fi lm. At the same time, the ‘contact’ area is bigger, so the risk of damage is much lower. To conclude: the higher the speed, the higher the nominal power; and the higher the DC Voltage, the greater the risk of damage.Th e damaged ball, with its eroded surface, will cause permanent vibrations with the inner and outer ring of the bearing; the typical pattern of a damaged ring is a consequential and visible eff ect.

Dominant featuresTh ere are diff erent ways of damaging a motor and/or a load bearing. Depending on the motor power, other dominant features are determined. For motors with a nominal power larger than 100kW, high frequency circulating currents and high frequency shaft ground-ing currents are the damaging elements, whereas for

smaller motors, capacitive discharge currents may lead to damage. Th e following section attempts to roughly explain the diff erent kinds of currents.

High frequency circulation currentsTh e high frequency common mode current (see dia-gram, above-left ) induces via the air gap a transient voltage between the shaft ends , developing a compen-sating current. Th e bearings and motor frame providea path for the current. If the voltage is high enough, the dielectric strength of the bearing lubrication fi lm may be overcome and a discharge current fl ows.

High frequency shaft grounding currentsDue to asymmetrical un-shielded motor cable and poor stator grounding, a high frequency stator voltage is created between the motor frame and ground (see diagram, below-left ), and therefore a current can fl ow through the motor bearing and shaft towards the load machine, and then to ground √. Th is current fl owing through the shaft can, depending on drive power and installation setup, damage the load bearing.

Th erefore, by using a frequency converter without a sinus fi lter at the output, the motor cables have to be three-phase and shielded type. Th e shield has to be con-nected on both motor and converter sides. Th is enables the common mode current to fl ow back on a defi ned path to its source.

Capacitive discharge currentsTh e air gap capacity acts as a voltage divider of the com-mon mode voltage. Th is result in a voltage coupled between the shaft and motor frame. Th e air gap capacitor discharges through the motor bearing to ground. Due to mechanical design, this phenomenon is mainly present in small motors below 100kW, typically below 30kW.

Preventing high frequency bearing currentsFor drives equipped with a sinus fi lter, there are no special actions to be consid-ered in terms of high frequency circulation currents. In other cases, most mistakes are simple installation issues:• Cable: For motor cables always use shielded three-phase

globalcement MAGAZINE October 2007 5

Above: High frequency circula-

tion currents.

Common mode current loop:

AC drive, cabling motor and PE

Mutual inductance coupling

between stator and rotor circuits

Circulating current through

shaft, bearings and motor frame.

Below: High frequency shaft

grounding currents, due to asym-

metrical un-shielded motor cable

and poor stator grounding.

cable. Never install single phase or un-shielded cables. Th e shield gives possible common mode currents a defi ned way back to the source (converter) without passing through the bearings.• du/dt: As modern frequency converters generate a high du/dt, its is recommended that the converter be equipped with a du/dt fi lter at the output, to prevent high frequency circulation currents. As this is only a big issue in motor power above 100kW, frequency converters for motors below this power don’t normally need to be equipped with a du/dt fi lter.• Breaking the current path: By insulating a bearing, the current path can be interrupted. It has to made certain that the current does not then take another route to ground via the load bearing. As mentioned above, a shielded motor cable is essential in providing defi ned current paths.

Diagnosis and protectionFor the correct functioning of the bearing, two condi-tions are important: to supervise the vibration and the temperature of the bearing. If both of these parameters are within the specifi ed ranges, the chances of being confronted with bearing problems are small.

VibrationVibrations of diff erent magnitudes can be detected on all rotating equipment. Vibration can be measured in three ways: displacement (the actual distance the object moves, usually measured in mm); acceleration (a part that is moving from rest, speeding up, slowing down and stopping twice per cycle, is accelerating and decel-erating continuously. Acceleration is measured in m/s2); velocity (the speed at which the object moves, measured in mm/s). Acceleration and velocity are constantly changing. One can measure a peak value of either, but a mean value oft en gives a better indication of the forces involved. Most instruments give the RMS value.

In terms of vibration, one of the aims is to protect the drive from too high or destructive a vibration, and the other is to know the magnitude and frequency spec-trum of the vibration. Th e less expensive supervision/protection mechanism is oft en used for smaller motors; this is mounted on the bearing housing, and produces a trip or a warning when the vibration exceeds it limit. Equipment like this does not show any frequency spec-trum. For bigger motors, a vibration measuring and protection system includes a sensor which shows the magnitude of the vibration across its entire spectrum. With this sophisticated system, the bearing condition can be detected long before the bearing becomes a problem. Retrieval of data can be done by hand held portable equipment, or by a permanently-installed system. Data is collected at certain predefi ned points on the bearing.

Shock Pulse measuring (SPM)Motors above a certain size, and those using anti-friction bearings, are equipped with SPM (shock pulse measuring) points on both sides of the motor, at the drive end and the non drive end. Th e condition of the bearing is measured and checked using the shock pulse method.

A shock pulse transducer produces a large ampli-tude oscillation from the weak shock pulse, because it is excited at its resonance frequency of 32kHz. Low motor bearing frequencies are fi ltered out. Th e fi ltered transducer signal refl ects the pressure variation in the rolling interface of the bearing. When the oil fi lm is thick, the shock pulse level is low, without distinc-tive peaks. Th e level increases when fi lm thickness is reduced. Bearing damage causes strong pulses at ir-regular intervals.

Spectrum analysisA more exact and detailed vibration signal analysis is provided by the method of spectrum analysis. In spec-trum analysis, speed and acceleration spectrums are usually followed, and are calculated using the math-

6 globalcement MAGAZINE October 2007

Bearing currents in a nutshellThere are three main types of current, which can be classifi ed as follows:

High Frequency shaft grounding currents

Asymmetric, unshielded motor cablingIncidental grounding of the motor shaft

through the gearbox or driven machineryPoor stator grounding

Capacitive discharge current

Special cases with small motorsMotor frame groundedShaft is not grounded via machinery

High frequency circulation current

Risk in medium and high power motors, frame sizes IEC 315 and up, PN >100 kW.

High du/dt and high switching frequency increase the risk.

Problem in > 95 % of cases

Conclusions

The majority of current damages are caused by circulating currentsThe shaft grounding current is usually related to improper cabling or groundingCapacitive discharge currents may be a problem with small motorsSelecting the correct cable type is importantCable shield and PE-conductor connections help to prevent damagesMotor/driven machinery installation can infl uence the location of the damage.

ematical Fourier series. Alarm limits can be specifi ed based on the spectrum.

Th e Fourier method allows any complex wave-form to be separated into simple sinusoidal waveform components. As the sine waves are separated from the combined waveform, they are converted to vertical peaks along the frequency axis, with a height deter-mined by their amplitude.

Usually the analyser forms the spectrum by calcu-lating them mathematically with the help of FFT (fast Fourier transform), a microprocessor algorithm that transforms the incoming signal from the analogue world (the time domain) into the frequency domain.

Bearing temperatureIf you want to be rid of bearing problems, the bearing temperature should be one of your key concerns. If the bearing temperature exceeds its pre-defi ned limits, whether due to ambient conditions or heat generated within the bearing itself, it has the potential to harm the bearing. Overheating in electric motor bearings is oft en lubricant related.

To prevent harm to the bearings, their temperature is continuously monitored and, depending on the plant visualisation confi guration, also displayed. Th e data col-lection is made with RTD probes, which are fi tted near the outer ring of the anti-friction bearings, and have to be of a four wire design. Only one probe is needed for each bearing.

Bearings of the future: the magnetic bearing?Drastic changes are expected in bearing technology. Some applications are moving in the direction of mag-netic bearing systems. Th e magnetic system is a new, non-contact method of supporting the rotor.

Being non-contact, this technology has negligible friction losses, no wear and, in another big step forward, can achieve high surface speeds. Besides this, it does not need any lubrication. Th is opens up possible ap-plications in areas such as vacuum operation, or those sensitive to contamination.

As the air gap between the two magnetic parts decreases, the attractive forces increase; therefore, elec-tromagnets are inherently unstable. A control system is needed to regulate the current and balance the forces, stabilising the position of the rotor. With magnetic bearings, vibration due to imbalance of rotating parts can be virtually eliminated and the shaft centre can be located within a micron-sized orbit.

Th e lifetime of magnetic bearings is expected to exceed 25 years, since there is no wear in the contact-free system, and deterioration is limited. It is important to note that magnetic bearings provide consistent performance throughout their life. Magnetic bearings do not wear. As a result, mechanical maintenance on the bearings themselves is eliminated; however, preventative maintenance of auxiliary components, electri-cal connections, etc. has to be done.

In order to form the magnetic fi eld, electric power is needed. Th e power consumption is mainly due to resistive losses in the coils. Th e amount of power varies for each size of bearing and its current rat-ing.

Everything looks fantastic, ex-cept when it comes to the question of what happens when the power fails. If power to the magnetic bearings is interrupted, the rotor will de-levitate. Consequently, magnetic bearings are fi tted with auxiliary bearings (roller or sleeve bearings), which are designed to withstand a number of full speed de-levitations, as required by each specifi c application. For enhanced reliability, the system can be backed up with an uninterruptible power supply (UPS), which will provide the power necessary to sup-port the shaft during coast down.

It is not expected that this type of bearing will be applied in the cement industry in the very near future. But, in the medium and long term, the magnetic bear-ing may play a certain role.

ConclusionsPlants with no bearing problems at all are as rare as ‘chicken with teeth.’ In all plants, for very diff erent rea-sons, bearing problems can and will occur. Th e aim of this paper was to describe the most common reasons for bearing problems, but also to discuss how to minimise those which are most avoidable, and the consequential sleepless nights because of unplanned plant shut-downs.

Some of the causes of bearing problems can cer-tainly be averted by avoiding the mistakes we have mentioned. But another class of bearing problems that can cause shut downs, can be easily avoided by recog-nising and interpreting abnormalities in the bearings at an early stage. It is of utmost importance to move away from reaction-based maintenance and repair, and toward a preventive approach. Bearing problems don’t just occur, they are allowed to develop.

globalcement MAGAZINE October 2007 7

Above: A damaged ball with an

eroded surface will vibrate with

the inner and outer ring of the

bearing, leading to the typical

pattern of a damaged outer ring.

Below: The capacitor created

within a bearing, between the

inner / outer race and the ball.

ABB Switzerland LtdCH-5405 Baden 5 DättwilSwitzerland

Phone +41 58 586 8444Fax +41 58 586 7333E-Mail process.industries@ch.abb.com

www.abb.com/cement 3BH

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